Breathing masks, or facepieces, have an inlet comprising a check valve to prevent exhalation to the breathing gas supply and to lower the total volume inside the mask. The check valve is ordinarily situated in the inlet and is larger in diameter than the hose fitting in order to minimize breathing resistance. Low pressure air or other breathing gas is provided to the mask either on a demand, pressure demand, or constant flow basis. Typically, breathing air is supplied (self-contained breathing apparatus or airline respirators) or is purified ambient air (gas masks, cartridge respirators, powered air purifying respirators).
A coupling that can be easily connected and disconnected is required to connect the disparate hose air-purifying or canister fitting to the mask inlet. A presently used coupling, similar to a pipe union, has a threaded nut that rotates freely around a reduced-diameter central proportion of a coupling body. The body is formed from two parts that must be bonded together (e.g., sonic bonded for synthetic resins) which in turn necessitates strength and leak testing of the bonded body to assure its reliability.